Funds are requested to support the SECOND INTERNATIONAL SYMPOSIUM ON THE BIOLOGY SEX DETERMINATION IN THE VERTEBRATES, April 10-14, 2000. The first symposium held in 1997 was the only meeting in which sex determination in all classes of vertebrates had been discussed in a single forum. It was a remarkable success. Participants from 17 different countries presented their research findings and the proceedings were published (J. Exptl. Zool. 281(5):357- 528, 1998). There was an overwhelmingly positive response to holding a second meeting in three years (see appendix). Since the discovery of the sex determining gene on the Y chromosome, SRY, in 1990 there have been many additional genes implicated in the process of sex determination. Homologues of a number of these genes (SF-1, SOX9, MIH, DAX1, WT1) have since been shown to be expressed in the differentiating gonads of fish, amphibians, reptiles and birds. SRY, however, appears to be involved in sex determination only in the placental mammals, but even among the mammals there are species lacking Sry, and the mechanism of action of SRY in humans is believed to differ from the mechanism of action of Sry in mice. The theme of the meeting will be on the evolution of the genes involved in sex determination, but it will also include topics of medical interest including sessions on mutations in SRY, SOX9, DMRT1 and subsequent disruption of sexual differentiation. Unusual mammalian species that exhibit different modes of sex determination will also be included. In addition some contributions on brain sex differentiation will be presented, plus a session on the interaction of transcription factors such as WT1, SF-1, DAX-1, SOX9, with each other and with SRY and MIH, and their role in sex determination. There will also be some papers on the effects of endocrine disruptors on sex determination. The meeting will be of interest to geneticists, developmental biologists, evolutionary biologists, environmental scientists, reproductive biologists, endocrinologists and clinicians.